class Solution {
    public String convert(String s, int numRows) {
        int n= s.length();
        if(numRows==1) return s;
        int d=2*numRows-2;
        StringBuilder ret = new StringBuilder();
        for(int i=0;i<n;i+=d) ret.append(s.charAt(i));
        for(int k=1;k<numRows-1;k++){
            for(int i=k,j=d-i;i<n||j<n;i+=d,j+=d){
                if(i<n) ret.append(s.charAt(i));
                if(j<n) ret.append(s.charAt(j));
            }
        }
        for(int i=numRows-1;i<n;i+=d) ret.append(s.charAt(i)); 
        return ret.toString();
    }
}